System and method for reducing iron oxide to metallic iron using coke oven gas and oxygen steelmaking furnace gas

ABSTRACT

A process for reducing iron oxide to metallic iron using coke oven gas (COG), including: a direct reduction shaft furnace for providing off gas; a COG source for injecting COG into a reducing gas stream including at least a portion of the off gas; and the direct reduction shaft furnace reducing iron oxide to metallic iron using the reducing gas stream and injected COG. The COG has a temperature of about 1,200 degrees C. or greater upon injection. The COG has a CH 4  content of between about 2% and about 13%. Preferably, the COG is reformed COG. Optionally, the COG is fresh hot COG. The COG source includes a partial oxidation system. Optionally, the COG source includes a hot oxygen burner.

CROSS-REFERENCE TO RELATED APPLICATION

The present non-provisional patent application/patent is acontinuation-in-part of U.S. patent application Ser. No. 13/107,013(nowU.S. Pat. No. 8,496,730), filed on May 13, 2011, and entitled “SYSTEMAND METHOD FOR REDUCING IRON OXIDE TO METALLIC IRON USING COKE OVEN GASAND OXYGEN STEELMAKING FURNACE GAS,” which claims the benefit ofpriority of U.S. Provisional Patent Application No. 61/334,786, filed onMay 14, 2010, and entitled “SYSTEM AND METHOD FOR REDUCING IRON OXIDE TOMETALLIC IRON USING COKE OVEN GAS AND OXYGEN STEELMAKING FURNACE GAS,”the contents of both of which are incorporated in full by referenceherein.

FIELD OF THE INVENTION

The present invention relates generally to a novel system and method forreducing iron oxide to metallic iron in an integrated steel mill or thelike that has a coke oven and/or an oxygen steelmaking furnace. Morespecifically, the present invention relates to a novel system and methodfor reducing iron oxide to metallic iron using coke oven gas and oxygensteelmaking furnace gas.

BACKGROUND OF THE INVENTION

Integrated steel mills and the like typically have coke ovens and/oroxygen steelmaking furnaces and use excess associated gases for heatingand power generation. In many applications, it would be desirable to usethe associated coke oven gas (COG) and/or the associated basic oxygenfurnace gas (BOFG) to reduce iron oxide to metallic iron, in the form ofdirect reduced iron (DRI), hot direct reduced iron (HDRI), or hotbriquetted iron (HBI). Both COG and BOFG contain significant percentagesof carbon monoxide (CO) and hydrogen (H₂), which are the primaryreductants for reducing iron oxide to metallic iron. The COG alsocontains 20+% methane (CH₄), which, under the proper conditions, may bereformed with carbon dioxide (CO₂) and water (H₂O) to form CO and H₂.BOFG may contain up to 20% nitrogen (N₂), which may build up to veryhigh levels in a recirculating system, for example.

BRIEF SUMMARY OF THE INVENTION

In various exemplary embodiments, the present invention provides aneconomical process for the direct reduction of iron ore when theexternal source of reductants is one or both of COG and BOFG, the latteralso known as oxygen steelmaking furnace gas. CO₂ is removed from amixture of shaft furnace off gas, obtained from a conventional directreduction shaft furnace, well known to those of ordinary skill in theart, and BOFG. This CO₂ lean gas is then mixed with clean COG,humidified, and heated in an indirect heater. Oxygen (O₂) is theninjected into the heated reducing gas to further increase itstemperature. This hot reducing gas flows to the direct reduction shaftfurnace, where CH₄ in the hot reducing gas undergoes reforming bycontact with the DRI/HDRI, followed by reduction of the iron oxide. Thespent hot reducing gas exits the direct reduction shaft furnace as shaftfurnace off gas, produces steam in a waste heat boiler, is cleaned in acooler scrubber, and is compressed and recycled to join fresh BOFG. Aportion of the shaft furnace off gas is sent to the heater burners.

Other contemplated uses for the BOFG include as a supplement to thecleaned/cooled shaft furnace off gas for use as the top gas fuel for theindirect heater. Similarly, the COG may be used for a variety of otherpurposes as well. The COG that is heated in the indirect heater ispreferably first cleaned of complex hydrocarbons that would foul theindirect heater via oxidation processing (i.e. partial combustion) orthe like (thereby correspondingly reducing, and potentially eliminating,the need for BOFG supplementation). COG with or without the complexhydrocarbons may also be used to supplement the top gas fuel for theindirect heater, as direct reduction shaft furnace transition zoneinjection gas, and/or to enrich the ultimate reducing gas stream. All ofthese possibilities, which are not mutually exclusive and may be used inany combination, are described in greater detail herein below.

One object of the present invention is to maximize the amount of DRI,HDRI, or HBI that may be produced from a given quantity of COG and/orBOFG.

Another object of the present invention is to provide an efficientprocess given varying quantities of COG and/or BOFG.

A further object of the present invention is to minimize equipment, andhence, plant cost by eliminating an external catalytic reformer, whichwould be used to generate CO and H₂ by reforming CH₄ in the COG withoxidants from the shaft furnace off gas and BOFG. Heating the mixture ofCO₂ lean gas, CO₂ lean BOFG, and COG in an indirect heater followed byO₂ injection and reforming in the direct reduction shaft furnace is lessexpensive than the use of the external catalytic reformer.

A still further object of the present invention is to allow theoperation of the direct reduction shaft furnace at a lower pressure thanwould otherwise be allowable, as the CH₄ level in the hot reducing gasdelivered to the direct reduction shaft furnace is lowered by adding theBOFG.

A still further object of the present invention is to limit the buildupof N₂ to an acceptable level by utilizing a portion of the spent hotreducing gas as indirect heater fuel.

In one exemplary embodiment, the present invention provides a novelsystem for reducing iron oxide to metallic iron using coke oven gas(COG) and oxygen steelmaking furnace gas (BOFG), including: a directreduction shaft furnace for providing off gas; a BOFG source forproviding BOFG; a carbon dioxide (CO₂) removal system for removing CO₂from a mixture of the off gas and the BOFG; a COG source for mixing aresulting CO₂ lean gas with COG; and the direct reduction shaft furnacereducing iron oxide to metallic iron using a resulting reducing gas. Thesystem also includes a saturator for adjusting the moisture content ofthe resulting reducing gas prior to it being used in the directreduction shaft furnace. The system further includes an indirect heaterfor heating the resulting reducing gas prior to it being used in thedirect reduction shaft furnace. Optionally, a fuel gas for the indirectheater comprises a portion of the off gas and a portion of one or moreof the COG and the BOFG. The system still further includes an oxygensource for adding oxygen to the resulting reducing gas prior to it beingused in the direct reduction shaft furnace. Optionally, the system stillfurther includes a conduit for communicating a portion of the COG fromthe COG source to the resulting reducing gas prior to it being used inthe direct reduction shaft furnace. Optionally, the system still furtherincludes a conduit for communicating a portion of the COG from the COGsource to a transition zone of the direct reduction shaft furnace.Optionally, the system still further includes a partial oxidationreactor for removing complex hydrocarbons from the COG prior to it beingmixed with the CO₂ lean gas. Preferably, an amount of the BOFG used isdependent upon an amount and composition of the COG used.

In another exemplary embodiment, the present invention provides a novelmethod for reducing iron oxide to metallic iron using coke oven gas(COG) and oxygen steelmaking furnace gas (BOFG), including: obtainingoff gas from a direct reduction shaft furnace; obtaining BOFG from aBOFG source; removing carbon dioxide (CO₂) from a mixture of the off gasand the BOFG; mixing a resulting CO₂ lean gas with COG from a COGsource; and reducing iron oxide to metallic iron in the direct reductionshaft furnace using a resulting reducing gas. The method also includesadjusting the moisture content of the resulting reducing gas using asaturator prior to it being used in the direct reduction shaft furnace.The method further includes heating the resulting reducing gas using anindirect heater prior to it being used in the direct reduction shaftfurnace. Optionally, a fuel gas for the indirect heater comprises aportion of the off gas and a portion of one or more of the COG and theBOFG. The method still further includes adding oxygen to the resultingreducing gas using an oxygen source prior to it being used in the directreduction shaft furnace. Optionally, the method still further includescommunicating a portion of the COG from the COG source to the resultingreducing gas using a conduit prior to it being used in the directreduction shaft furnace. Optionally, the method still further includescommunicating a portion of the COG from the COG source to a transitionzone of the direct reduction shaft furnace using a conduit. Optionally,the method still further includes removing complex hydrocarbons from theCOG prior to it being mixed with the CO₂ lean gas using a partialoxidation reactor. Preferably, an amount of the BOFG used is dependentupon an amount and composition of the COG used.

In a further exemplary embodiment, the present invention provides amethod for reducing iron oxide to metallic iron, including: obtainingoff gas from a direct reduction shaft furnace; obtaining basic oxygenfurnace gas (BOFG) from a BOFG source; removing carbon dioxide (CO₂)from a mixture of the off gas and the BOFG; and reducing iron oxide tometallic iron in the direct reduction shaft furnace using a resultingCO₂ lean gas. Optionally, the method also includes mixing the resultingCO₂ lean gas with coke oven gas (COG) from a COG source prior to usingit as a reducing gas. Optionally, the method further includes removingcomplex hydrocarbons from the COG prior to it being mixed with theresulting CO₂ lean gas.

In a still further exemplary embodiment, the present invention providesa method for reducing iron oxide to metallic iron, including: obtainingoff gas from a direct reduction shaft furnace; mixing the off gas withcoke oven gas (COG) from a COG source; and reducing iron oxide tometallic iron in the direct reduction shaft furnace using a resultingreducing gas. Optionally, the method also includes: obtaining basicoxygen furnace gas (BOFG) from a BOFG source; removing carbon dioxide(CO₂) from a mixture of the off gas and the BOFG; and mixing a resultingCO₂ lean gas with the COG from the COG source. Optionally, the methodfurther includes removing complex hydrocarbons from the COG prior to itbeing mixed with the CO₂ lean gas.

In a still further exemplary embodiment, the present invention providesa system for reducing iron oxide to metallic iron using coke oven gas(COG), including: a direct reduction shaft furnace for providing offgas; a COG source for injecting COG into a reducing gas stream includingat least a portion of the off gas; and the direct reduction shaftfurnace reducing iron oxide to metallic iron using the reducing gasstream and injected COG. The COG has a temperature of about 1,200degrees C. or greater upon injection. The COG has a CH₄ content ofbetween about 2% and about 13%. Preferably, the COG is reformed COG.Optionally, the COG is fresh hot COG. The COG source includes a partialoxidation system. Optionally, the COG source includes a hot oxygenburner. Optionally, the system still further includes a basic oxygenfurnace gas (BOFG) source for injecting BOFG into the off gas that formsat least a portion of the reducing gas stream. Optionally, the systemstill further includes a carbon dioxide (CO₂) removal system forremoving CO₂ from the mixture of the off gas and the BOFG.

In a still further exemplary embodiment, the present invention providesa method for reducing iron oxide to metallic iron using coke oven gas(COG), including: providing a direct reduction shaft furnace forproviding off gas; providing a COG source for injecting COG into areducing gas stream including at least a portion of the off gas; and thedirect reduction shaft furnace reducing iron oxide to metallic ironusing the reducing gas stream and injected COG. The COG has atemperature of about 1,200 degrees C. or greater upon injection. The COGhas a CH₄ content of between about 2% and about 13%. Preferably, the COGis reformed COG. Optionally, the COG is fresh hot COG. The COG sourceincludes a partial oxidation system. Optionally, the COG source includesa hot oxygen burner. Optionally, the method still further includesproviding a basic oxygen furnace gas (BOFG) source for injecting BOFGinto the off gas that forms at least a portion of the reducing gasstream. Optionally, the method still further includes providing a carbondioxide (CO₂) removal system for removing CO₂ from the mixture of theoff gas and the BOFG.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated and described herein with referenceto the various drawings, in which like reference numbers are used todenote like system components/method steps, as appropriate, and inwhich:

FIG. 1 is a schematic diagram illustrating one exemplary embodiment ofthe novel system and method for reducing iron oxide to metallic ironusing COG and/or BOFG of the present invention;

FIG. 2 is a schematic diagram illustrating one exemplary embodiment of aprocess for removing complex hydrocarbons from the COG in conjunctionwith the system and method of FIG. 1; and

FIG. 3 is a schematic diagram illustrating an alternative exemplaryembodiment of the novel system and method for reducing iron oxide tometallic iron using COG of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring specifically to FIG. 1, in one exemplary embodiment, the novelsystem and method for reducing iron oxide to metallic iron using COGand/or BOFG (system and method collectively 5) of the present inventionincludes individual components that are well known to those of ordinaryskill in the art, and thus they are not illustrated or described inexcessive detail herein, but that are combined together in an inventiveprocess. These components include, but are not limited to, aconventional direct reduction shaft furnace 10, a waste heat boiler 18,a cooler scrubber 20, a BOFG source 30 (and/or appropriate storagevessel), a CO₂ removal system 40, a COG source 50 (and/or appropriatestorage vessel), a saturator 60, an indirect heater 70, and an oxygensource 80 (and/or appropriate storage vessel).

The direct reduction shaft furnace 10 has an upper end where iron ore inthe form of pellets, lumps, aggregates, etc. 14 is fed. The reducedpellets, lumps, aggregates, etc. 14 are removed at a lower end 13 of thedirect reduction shaft furnace 10 as DRI. A reducing gas inlet conduit15 is located between the feed charge and the product discharge, andsupplies hot reducing gas to the direct reduction shaft furnace 10. Thishot reducing gas contains CH₄, which is reformed near the gas inletsection of the direct reduction shaft furnace 10 by CO₂ and H₂Ocontained in the hot reducing gas to produce additional CO and H₂. TheHDRI acts as a catalyst in the reforming reaction. Following thisreforming reaction, the hot reducing gas containing CO and H₂ reducesthe iron oxide to metallic iron and exits the direct reduction shaftfurnace 10 as spent reducing gas through an offtake conduit at the topof the direct reduction shaft furnace 10 flowing into a duct 17 to thewaste heat boiler 18, and then to the cooler scrubber 20. The steamgenerated in the waste heat boiler 18 provides the majority of theregeneration heat for the CO₂ removal system 40, for example. The coolerscrubber 20 cools and cleans the spent off gas, which exits the coolerscrubber through a conduit 21.

Next, a portion of the cooled off gas enters another conduit 23 andflows to the burners of the indirect heater 70. A portion of the cooledoff gas also enters a further conduit 22 and joins a conduit 32 from theBOFG source 30, forming another conduit 34 that flows to a compressor35. The compressed gas from the compressor 35 flows to the CO₂ removalsystem 40, where CO₂ is scrubbed from the gas. The CO₂ lean gas in theconduit 41 is then enhanced by COG from another conduit 52, and thenenters a further conduit 56, which flows to the saturator 60 where H₂Ois added to the gas in order to adjust it for carbon control in thedirect reduction shaft furnace 10.

Additional BOFG is combined directly with the top gas fuel streamthrough a conduit 33. Additional COG is sent to the auxiliary burners ofthe indirect heater 70 through one or more conduits 53 and 54 and to thetransition zone of the direct reduction shaft furnace 10, as transitionzone injection gas, through one or more other conduits 53 and 55. Thegas from the saturator 60 flows through a conduit 61 to the indirectheater 70, where the gas is heated to near reduction temperature by theburners fueled by the combination of spent direct reduction furnace offgas and BOFG, as well as the auxiliary burners fueled by COG, forexample.

Combustion air is preheated by heat exchange with heater flue gas. Thehot gas from the indirect heater 70 leaves through a conduit 71 and O₂from the oxygen source 80 is added via another conduit 81 to raise thetemperature of the gas to 1000 degrees C. or higher. The gas then flowsthrough a further conduit 15, with the elevated temperature required tosupply the endothermic load necessary for the in situ reforming in thereduction shaft furnace 10.

In general, COG and BOFG have analyses that may vary depending on theparticular raw materials and specific practices at various steel millsthroughout the world. The table below provides some non-limitingexamples:

COG BOFG CO 6-7 55-72 CO₂ 1-2 13-18 H₂ 61-63 1-4 H₂O 1-5 1-5 CH₄ 21-241-3 N₂ 3-7 11-20

If the COG and BOFG are utilized in the most efficient manner to produceDRI/HDRI/HBI with a minimum amount of COG and/or BOFG without exportfuel, there is a specific ratio of COG to BOFG for each analysis of thegases. This ratio may vary from about 0.95 to about 1.25. For BOFG withhigher amounts of CO, and consequently lower amounts of N₂, the ratio iscloser to 0.95. For BOFG with higher amounts of N₂, and consequentlylower amounts of CO, more COG is required and the ratio is closer to1.25.

As mentioned above, it is possible to run varying ratios of COG to BOFGoutside of the calculated best operating point, but it must be done withexport fuel that would have to be consumed elsewhere. One such use ofthis export fuel could be to raise additional steam for regeneration inthe CO₂ removal system 40, for example.

As described above, in addition to supplementing the shaft furnace offgas stream and contributing to the eventual reducing gas stream, othercontemplated uses for the BOFG include supplementing the shaft furnaceoff gas stream for use as the top gas fuel for the indirect heater 70(via conduits 31, 33, and 24). Similarly, in addition to supplementingthe shaft furnace off gas stream and contributing to the eventualreducing gas stream, the COG may be used for a variety of other purposesas well.

Referring specifically to FIG. 2, the COG from the COG source 50 that iseventually heated in the indirect heater 70 (FIG. 1) is preferably firstcleaned of sulfur and complex hydrocarbons that would foul the indirectheater 70 via oxidation processing (i.e. partial combustion) or the likein a partial oxidation reactor 90 or the like, with the addition of O₂and H₂O (i.e. steam). This cleaning process correspondingly reduces, andpotentially eliminates, the need for BOFG supplementation, if sodesired. The cleaning process is primarily required to deal with thepresence of quantities of NH₃, H₂S, Tars, HCN, Naphthalene, and BTX(Benzol, Toluene, and Xylene) in the COG. Optionally, the cleaningprocess takes place as a lesser reaction in the ducts of the reducinggas system, as opposed to the partial oxidation reactor 90. Theoxidation reaction looks as follows (exemplary only):

-   -   COG—7.5% CO, 3.5% CO₂, 54% H₂, 25.25% CH₄, 7.45% N₂, 2.3%        C_(n)H_(m)    -   1 Part Steam to 10 Parts COG    -   Oxygen Addition for 10 Parts COG:    -   1.7 Parts Oxygen:        -   21.38% CO, 2.8% CO₂, 61.16% H₂, 7.28% H₂O, 2.91% CH₄, 4.46%            N2 Temp. 800 degrees C., 17.1 Parts Product Gas    -   2 Parts Oxygen:        -   22.81% CO, 2.54% CO₂, 61.74% H₂, 8.14% H₂O, 0.49% CH₄, 4.27%            N2 Temp. 880 degrees C., 17.9 Parts Product Gas

Referring again specifically to FIG. 1, COG with or without the complexhydrocarbons may also be used to supplement the top gas fuel for theindirect heater 70 (via conduits 53 and 54), as direct reduction shaftfurnace transition zone injection gas (via conduits 53 and 55), and/orto enrich the ultimate reducing gas stream (via conduits 53, 54, and59). Each of these possibilities is not mutually exclusive and all ofthese possibilities may be used in any combination.

Referring now to FIG. 3, in an alternative exemplary embodiment of thepresent invention, reformed COG 100 is injected 102 into thesystem/process stream 15 just prior to the direct reduction shaftfurnace 10. Preferably, this COG 100 is reformed COG, as indicatedpreviously, or fresh hot COG, and is from a partial oxidation system,such as a hot oxygen burner (which injects COG 90 into an ultra-hotflame), well known to those of ordinary skill in the art. The reformedCOG 100 is hot (between about 1000 degrees C. and about 1600 degrees C.)and is injected 102 into the about 900 degrees C. stream 15. Because ofthis heat, the oxygen 80 injection 81 described previously (see FIG. 1)becomes optional. The result is less oxygen 80 injection 81 into thesystem/process 5, while still avoiding the development of carbon soot.This COG 100 injection 102 may be used in place of, or as a complementto, the cooler COG and/or BOFG injection sources and points describedpreviously. For example, the COG 100 injection 102 may be used inconjunction with a standard Midrex natural gas process with a reformer.As such, the previously described CO₂ removal system 40 and indirectheater 70 would not be necessary (the reformer would adequately performboth of these functions).

The reformed COG 100 has the following exemplary contents: 2-13% CH₄ (atabout 1,500 degrees C.-about 1,200 degrees C., respectively), 18.7% CO,1.7% CO₂, 43.4% H₂, 17.7% H₂O, 3.6% N₂, and 1.8% C₂H₆, and possibly 0.9%C₂H₄ and 1.7% C₂H₂. Of course these contents are exemplary only andshould not be construed as limiting in any respect.

Although the present invention has been illustrated and described hereinwith reference to preferred embodiments and specific examples thereof,it will be readily apparent to those of ordinary skill in the art thatother embodiments and examples may perform similar functions and/orachieve like results. All such equivalent embodiments and examples arewithin the spirit and scope of the present invention, are contemplatedthereby, and are intended to be covered by the following claims.

What is claimed is:
 1. A method for reducing iron oxide to metallic ironusing coke oven gas (COG), comprising: providing a direct reductionshaft furnace for providing off gas; providing a COG source forinjecting COG into a reducing gas stream comprising at least a portionof the off gas; and the direct reduction shaft furnace reducing ironoxide to metallic iron using the reducing gas stream and injected COG;wherein COG is also provided in a separate stream to a transition zoneof the direct reduction shaft furnace.
 2. The method of claim 1, whereinthe COG has a temperature of about 1,200 degrees C. or greater uponinjection.
 3. The method of claim 1, wherein the COG has a CH₄content ofbetween about 2 volume % and about 13 volume %.
 4. The method of claim1, wherein the COG comprises reformed COG.
 5. The method of claim 1,wherein the COG comprises fresh hot COG with a temperature of betweenabout 1,000 degrees C. and about 1,600 degrees C.
 6. The method of claim1, wherein the COG source comprises a partial oxidation system.
 7. Themethod of claim 1, wherein the COG source comprises an oxygen burner. 8.The method of claim 1, further comprising providing a basic oxygenfurnace gas (BOFG) source for injecting BOFG into the off gas that formsat least a portion of the reducing gas stream.
 9. The method of claim 8,further comprising providing a carbon dioxide (CO₂) removal system forremoving CO₂ from the mixture of the off gas and the BOFG.
 10. A methodfor reducing iron oxide to metallic iron, comprising: reducing the ironoxide to the metallic iron in a direct reduction shaft furnace using areducing gas that comprises a mixture of off gas from the directreduction shaft furnace and coke oven gas (COG) from a COG source,wherein COG is also provided in a separate stream to a transition zoneof the direct reduction shaft furnace.
 11. The method of claim 10,wherein the COG has a temperature of about 1,200 degrees C. or greaterupon mixing.
 12. The method of claim 10, wherein the COG has aCH₄content of between about 2 volume % and about 13 volume %.
 13. Themethod of claim 10, wherein the COG comprises reformed COG.
 14. Themethod of claim 10, wherein the COG source comprises an oxygen burner.15. A system for reducing iron oxide to metallic iron using coke ovengas (COG), comprising: a direct reduction shaft furnace for providingoff gas; a COG source for injecting COG into a reducing gas streamcomprising at least a portion of the off gas; and the direct reductionshaft furnace reducing iron oxide to metallic iron using the reducinggas stream and injected COG; wherein COG is also provided in a separatestream to a transition zone of the direct reduction shaft furnace. 16.The system of claim 15, wherein the COG has a temperature of about 1,200degrees C. or greater upon injection.
 17. The system of claim 15,wherein the COG has a CH₄content of between about 2 volume % and about13 volume %.
 18. The system of claim 15, wherein the COG comprisesreformed COG.
 19. The system of claim 1, wherein the COG comprises freshhot COG with a temperature of between about 1,000 degrees C. and about1,600 degrees C.
 20. The system of claim 15, wherein the COG sourcecomprises a partial oxidation system.
 21. The system of claim 15,wherein the COG source comprises an oxygen burner.
 22. The system ofclaim 15, further comprising a basic oxygen furnace gas (BOFG) sourcefor injecting BOFG into the off gas that forms at least a portion of thereducing gas stream.
 23. The system of claim 22, further comprising acarbon dioxide (CO₂) removal system for removing CO₂ from the mixture ofthe off gas and the BOFG.